The present invention relates to phase difference measuring apparatus for measuring the phase of a transmission circuit, such as an amplifier, a filter, a phase shifter, a transmission line or the like.
A conventional phase difference measuring apparatus is arranged as follows: A first signal obtained by passing a test signal through a transmission circuit under test and a second signal, which is the test signal having not been applied to the transmission circuit, are supplied as input signals and waveform-shaped into rectangular waves; one waveform-shaped output signal is polarity-inverted; the polarity-inverted signal and the other waveform-shaped output signal are ANDed with each other; the AND output is level-converted so that its peak value may take a predetermined value; the level-converted output is applied to a smoothing circuit (a low-pass filter circuit) to have its level averaged; and the averaged level is measured by a voltmeter. By calibrating the indication of the voltmeter in terms of phase, the phase difference between the first and second signal is indicated by the voltmeter. Such an arrangement of the prior art phase meter is briefly described in "Electron" at pages 47-48 published on Nov. 10, 1977.
This prior art phase difference measuring apparatus employs the smoothing circuit for obtaining the average level, but a desired degree of measurement accuracy cannot be obtained unless the time constant of the smoothing circuit is changed in accordance with the frequency of the signal (i.e. the input signal) to be measured. Accordingly, it is necessary to adopt such an arrangement that switches the time constant of the smoothing circuit in accordance with the measuring frequency range, which makes the measuring operation complex. The measurement accuracy is dependent upon the precision of an analog circuit, such as the level converter, the smoothing circuit or the like, and an expensive arrangement is needed for achieving measurement accuracy higher than 0.1.degree.. Further, since the phase difference is measured as a voltage value, the voltmeter is required.